Fix LP BUG#719198

Analysis:
The assert failed because the execution code for
partial matching is designed with the assumption that
NULLs on the left side are detected as early as possible,
and a NULL result is returned before any lookups are
performed at all.

However, in the case of an Item_cache object on the left
side, null was not detected properly, because detection
was done via Item::is_null(), which is not implemented at
all for Item_cache, and resolved to the default Item::is_null()
which always returns FALSE.

Solution:
Use the property Item::null_value instead of is_null(), which
is properly updated for Item_cache objects as well.

mysql-test/r/subselect_mat.result

@@ -1371,3 +1371,41 @@ SELECT pk FROM t1 WHERE (b,c,d) IN (SELECT b,c,d FROM t2 WHERE pk > 0);
 pk
 2
 DROP TABLE t1, t2;
+#
+# LPBUG#719198 Ordered_key::cmp_key_with_search_key(rownum_t): Assertion `!compare_pred[i]->null_value'
+# failed with subquery on both sides of NOT IN and materialization
+#
+CREATE TABLE t1 (f1a int, f1b int) ;
+INSERT IGNORE INTO t1 VALUES (1,1),(2,2);
+CREATE TABLE t2 ( f2 int);
+INSERT IGNORE INTO t2 VALUES (3),(4);
+CREATE TABLE t3 (f3a int, f3b int);
+set session optimizer_switch='materialization=on,partial_match_rowid_merge=on,partial_match_table_scan=off';
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	SUBQUERY	t3	system	NULL	NULL	NULL	NULL	0	const row not found
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
+f2
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	SUBQUERY	t3	system	NULL	NULL	NULL	NULL	0	const row not found
+SELECT * FROM t2 WHERE (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
+f2
+insert into t3 values (1,1),(2,2);
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3 where f3a > 3) NOT IN (SELECT f1a FROM t1);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3 where f3a > 3) NOT IN (SELECT f1a FROM t1);
+f2
+3
+4
+drop table t1, t2, t3;

mysql-test/t/subselect_mat.test

@@ -1011,3 +1011,31 @@ SELECT pk FROM t1 WHERE (a) IN (SELECT a FROM t2 WHERE pk > 0);
 SELECT pk FROM t1 WHERE (b,c,d) IN (SELECT b,c,d FROM t2 WHERE pk > 0);
 DROP TABLE t1, t2;
 
+--echo #
+--echo # LPBUG#719198 Ordered_key::cmp_key_with_search_key(rownum_t): Assertion `!compare_pred[i]->null_value'
+--echo # failed with subquery on both sides of NOT IN and materialization
+--echo #
+
+CREATE TABLE t1 (f1a int, f1b int) ;
+INSERT IGNORE INTO t1 VALUES (1,1),(2,2);
+CREATE TABLE t2 ( f2 int);
+INSERT IGNORE INTO t2 VALUES (3),(4);
+CREATE TABLE t3 (f3a int, f3b int);
+
+set session optimizer_switch='materialization=on,partial_match_rowid_merge=on,partial_match_table_scan=off';
+
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
+
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
+SELECT * FROM t2 WHERE (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
+
+insert into t3 values (1,1),(2,2);
+
+EXPLAIN
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3 where f3a > 3) NOT IN (SELECT f1a FROM t1);
+SELECT * FROM t2 WHERE (SELECT f3a FROM t3 where f3a > 3) NOT IN (SELECT f1a FROM t1);
+
+drop table t1, t2, t3;

sql/item.h

@@ -3342,6 +3342,13 @@ class Item_trigger_field : public Item_field,
 };
 
 
+/**
+  @todo
+  Implement the is_null() method for this class. Currently calling is_null()
+  on any Item_cache object resolves to Item::is_null(), which reutns FALSE
+  for any value.
+*/
+
 class Item_cache: public Item_basic_constant
 {
 protected:

sql/item_subselect.cc

@@ -5069,7 +5069,7 @@ bool subselect_rowid_merge_engine::partial_match()
   for (uint i= test(non_null_key); i < keys_count; i++)
   {
     DBUG_ASSERT(merge_keys[i]->get_column_count() == 1);
-    if (merge_keys[i]->get_search_key(0)->is_null())
+    if (merge_keys[i]->get_search_key(0)->null_value)
     {
       ++count_nulls_in_search_key;
       bitmap_set_bit(&matching_outer_cols, merge_keys[i]->get_keyid());